It is desired to provide a coated polyester film for the production of optically variable pigments. The coating for use in this process is preferred to be processed inline to provide for the lowest cost of production. Furthermore, this coating is defined to have the following unique and special attributes:
1) High thermal stability,
2) Very rapid dissolution into a recovery solvent like acetone,
3) Water insolubility,
4) Tg greater than 0 C, Tg less than 50 C as defined by the differential scanning calorimetery (DSC) method,
5) Highly extensible to survive an inline coating process, without turning hazy or white.
6) Substantially noncrosslinked,
7) Surface Energy  less than 40 dyne/cm and  greater than =35 dyne/cm.
Surprisingly, we have found that materials with these special attributes have excellent metallizing and recovery properties, making them amenable to the production of optically variable pigments.
U.S. Pat. No. 5,968,664 discloses a polyester film with a releasable coating comprising the following:
i) Copolymers of MethylMethacrylate (MMA) and EthylAcrylate (EA)
ii) Homopolymers of Polyacrylic Acid (AA)
iii) Said coating layers being soluble in the release solvent.
Our invention described herein is distinctly different than those mentioned in the ""664 patent in that:
a) The specific coating formulations required herein are different.
b) The limitations of thermal stability, quantification of acetone solubility, metal release, surface energy and glass transition temperatures are not disclosed in the reference.
U.S. Pat. No. 5,795,649 discloses a polymeric coated film with the following attributes:
i) Water soluble copolymer of styrene (S) and an alpha, beta-unsaturated carboxylic acid or anhydride like maleic anhydride,
ii) Molecular weight of said copolymer from about 700 to 10,000,
Our invention described herein is distinctly different than those mentioned in the ""649 patent in that:
a) The specific coating formulations required are different. Additionally, the molecular weight restriction of the referenced patent is considered to limit the actual processability of the materials.
b) The limitations of thermal stability, quantification of acetone solubility, metal release, surface energy and glass transition temperatures are not disclosed in the reference.
U.S. Pat. No. 5,928,781 discloses a method of making a thin layer or flakes of materials by using a coated substrate. The coating is further defined as:
i) Solvent soluble,
ii) Containing a Crotonic acid polymer.
The invention described herein is distinctly different than those mentioned in the ""781 patent in that:
a) The specific coating formulations required herein are different.
b) The important limitations of thermal stability, quantification of acetone solubility, metal release, surface energy and glass transition temperatures are not disclosed in the reference.
U.S. Pat. No. 6,013,370 discloses the method for making optically variable pigments by use of a sputtering process. The mode of cracking of the flake is indicated as being a key to the process.
The ""370 patent is a process patent. It discloses that the mode of cracking of the flake is a key to the usefulness of the flake itself. It discloses that the coating disclosed assists in producing flake of high aspect ratio, and mentions it as desirable.
Thermal stability in accordance with this invention is defined herein as the temperature at which a 10% weight loss occurs of the primary decomposition curve for the polymeric coating materials. In determining thermal stability, the coating material is heated in a nitrogen atmosphere from ambient to 105xc2x0 C. at 10xc2x0 C./min. The sample is then held at 105xc2x0 C. for 15 minutes then ramped to 550xc2x0 C. at 10xc2x0 C. per minute. The temperature corresponding to 10% weight loss is read from the weight loss curve.
Acetone solubility has been measured by immersing the coated film into a bath of acetone for approximately 2 seconds. The films were then removed and allowed to air dry. The films were then inspected with a light microscope at 40xc3x97 magnification. A rating of 5 means the coating was completely removed in the 2 second immersion in the acetone solution. A rating of 0 means that no coating was seen to have been removed via light microscopy inspection. A rating of 1 means that less than about 25% of the coating was removed via exposure in acetone. A rating of 2 means that less than about 50% of the coating was removed via exposure to acetone. A rating of 3 means that less than about 75% of the coating was removed by acetone. A rating of 4 means that less than about 100% was removed by acetone, or that a trace of the coating layer remained in the area immersed in acetone.
Surface energy is defined from measurements of the contact angle of water on the surface of the coated PET base film. The surface energy numbers reported here are derived from conversion of the contact angle of water on the surface to absolute wetting tension values.
Glass transition temperatures (Tg) are reported from manufacturers literature. They are obtained by making measurements by use of the well-known DSC method. Glass transition temperatures were measured by the manufacturers of the polymer emulsions via the use of differential scanning calorimetery (DSC). DSC is a well-known method in the art to characterize thermal phase transitions of polymeric materials; please see Allcock, H. R., Camp, J. W. xe2x80x9cContemporary Polymer Chemistryxe2x80x9d, Prentice-Hall 1990, Chapter 17, which describes DSC as the Industry Standard technique for measuring Tg values.
Metal release is measured from bell jar metallizing the coated PET or related polymeric samples with aluminum metal to an OD of about 2.5. The metallized samples are then submersed in acetone for approximately 2 seconds, removed, and then brushed gently with a dry cloth. A metal release value of Excellent means that essentially all aluminum was released when using this technique. A metal release rating of fair means that approximately 25-50% of the metal was released via this technique. A metal release value of poor means that less than approximately 25% was released by this technique.
Although not wishing to be bound by any particular theory, we believe the following description of the physical process employed is helpful in illustrating the invention.
In the vacuum sputtering process of polyester film (PET), an organic surface (the PET film) is in contact with a hot metal gas (the sputerant). The temperature of this gas may approach 1000xc2x0 C. When the hot metal gas contacts the organic coating, decomposition of the underlying substrate can often occur. This decomposition may result in crosslinking of the coating, shrinkage of the coating or off gassing of decomposed adducts. Crosslinking of the coating will render it insoluble and thus unable to release the inorganic pigment. Thermal shrinkage or off gassing of decomposition adducts will degrade the metal layer through increased cracking and pitting of the metal layer. Therefore, it is desired to construct a coating surface having high thermal stability to resist the effects of high thermal loads from the sputtering process.
Concurrently, metal having a high surface energy has stronger adhesion to surfaces having high surface energy. An increase in metal/coating adhesion forces increases the time required to release the sputter coated metal from the surface of the coated PET. Therefore, we have found that for best processing of the metalized films, that a surface energy SE between about 35 dyne/cm and about 40 dyne/cm is preferred.